Continuously variable transmissions (CVT's) are transmissions that change ratio continuously, not in discrete intervals. This continuous nature of CVT's gives them an infinite number of gear ratios, making them very attractive for automotive uses. One attractive aspect is that higher efficiency of the engine may result because it can be run at a desired speed for a broad range of vehicle speeds, allowing an efficient engine operating point for the specific power requirement.
Previously, there have been various methods to control the ratio of the CVT. The most straight forward way is to define a target CVT ratio in response to a particular parameter and then adjust the control valve of the CVT in a manner to achieve that ratio. Change in the CVT ratio may cause the operating point of the engine to change. The limitation of the above basic system is that there is no control over the speed/torque path followed by the engine when the target CVT ratio is being achieved. The speed/torque path of the engine between operating points is referred to as the engine transient response path.
Other prior engine-CVT control systems do not allow for variable response paths of the engine, resulting in no direct control over the engine response path. Still other systems achieve some direct control over transient response but fail to achieve complete control because there is no independent control over the speed and torque of the engine.
What is desirable in driving an engine-CVT system is independent control over the speed and torque of the engine. Independent control over engine speed and engine torque would allow for increased flexibility over the speed/torque response path of the engine. For example the transition between operating points of the engine could be done more efficiently, or could be done to provide increased engine torque output, or could be done in any variety of ways a system designer would choose. Furthermore, independent control of the engine speed and engine torque would allow for increased flexibility over the acceleration/deceleration response (transient response) of the vehicle. Previous systems fail to achieve independent control over engine speed and engine torque and, as a result, have limited control over the transient responses of the engine and vehicle.